The zebrafish is a relatively new and extremely powerful genetic system for studying vertebrate development (Driever et al. 1996). As zebrafish are small, >70 can be maintained in a five gallon tank. Each mother produces >400 eggs weekly. Hence, large-scale genetics can be performed with ease. At Children's Hospital we have pioneered use of the zebrafish system for hematopoiesis. Two large scale screens for developmental mutants have produced >50 independent mutants affecting blood cell formation (Ransom et. al. 1996). These mutants have been collected at Children's Hospital. Presently, this CORE is a repository for zebrafish mutants of hematopoiesis, and supports their characterization and distribution.